The present invention relates to a polyimide aligning film which excels in electrical properties and reliability for the field of electronic materials, in particular for a liquid crystal aligning film; to a polyamic acid composition for providing such an aligning film; and to a liquid crystal display element produced from such an aligning film.
The present mainstream of liquid crystal display elements is that utilizing nemetic liquid crystals. Liquid crystal display elements presently in practical use include a matrix TN element twisted by 90xc2x0, an STN element usually twisted by 180xc2x0 or more, and a TFT liquid crystal display element utilizing thin film transistors. In addition to these, IPS (In Plane Switching) mode having improved viewing angle properties has been developed. The progress of liquid crystal display elements is not limited to only these modes, but active efforts to improve peripheral materials have been made toward improvement of the properties of liquid crystal display elements.
With the wider use of liquid crystal display elements in various fields, liquid crystal display elements having improved properties have been demanded. Such demands include better display performance and higher reliability in long-time use. In particular, important factors for TFT liquid crystal display elements are electrical properties, especially the voltage holding ratio and the residual voltage. If the voltage holding ratio is low, the voltage applied to liquid crystals during the field period lowers, causing the problem of lowered contrast. Therefore, a higher voltage holding ratio is desired, and ideally, the voltage holding ratio should be 100%.
On the other hand, if the residual voltage is large, it will remain even after the electric field has been turned off after being once turned on. In such a case, images to be erased will remain as residual images. Therefore, it is desired that the residual voltage approach zero infinitesimally. This residual image phenomenon is one of the most important issues in liquid crystal display elements.
Another problem in liquid crystal display element is an increase in the electric current of the display element occurring during long-term use. The increase in the electric current lowers the voltage applied to the liquid crystal itself and raises liquid crystal molecules insufficiently, thus lowering contrast, and in some cases failing to display images.
Furthermore, in manufacturing processes for liquid crystal display elements, the stability of properties of the display elements is an important factor to achieve process margin. For example, if the pre-tilt angle of liquid crystals changes significantly with solvent evaporation conditions after coating of an alignment material, with imidizing (normally heating) conditions of a polyamic acid, or with annealing conditions after injection of liquid crystals, elements having a stable pre-tilt angle cannot be manufactured unless the process conditions are strictly controlled for each process.
When TFT elements were first marketed, the pre-tilt angle of liquid crystals for a TFT element was required to be nearly 0 degrees. In recent years, however, liquid crystals have been required to have some pre-tilt angle (3 to 12 degrees) in order to inhibit the occurrence of domains.
An object of the present invention is to provide a liquid crystal aligning film for producing a liquid display element having a pre-tilt angle required for TFT display elements, small manufacturing process dependence of the pre-tilt angle, little residual voltage of the liquid crystal display element, and high voltage holding ratio, as well as high reliability.
The prior art for producing an element having a small residual voltage and a high voltage holding ratio suited for TFT display elements is disclosed in, for example, Japanese Patent Application Laid-open No. 8-106097. In this reference, a tetracarboxylic dianhydride having aromatic groups, aliphatic groups, or a combination of aromatic and aliphatic groups is used as a tetracarboxylic acid component, and a diamine having ether groups is used as a main diamine component, but a large effect cannot be obtained. The pre-tilt angle is as small as 1.3 to 3.6 degrees.
In Japanese Patent Application Laid-open No. 7-120768 is disclosed an alignment material containing a polyamic acid having an aliphatic tetracarboxylic dianhydride as its essential component and a polyamic acid having an aromatic tetracarboxylic dianhydride as its essential component. Although this reference describes that siloxane-based diamines have particularly excellent properties, experiments conducted by the present inventors show that the residual voltage is small but alignment is poor. Also, no diamine components that increase the tilt angle are described.
The inventors of the present invention conducted repeated examinations for solving the above problems, and found that the above object was achieved by a polyimide film used as a polymer component of a liquid crystal aligning film used in liquid crystal elements, which film is formed by heat-curing a polyamic acid composition comprising a polyamic acid that provides a polyimide resin giving a small residual voltage and a high voltage holding ratio, and a polyamic acid that provides a polyimide resin giving a large pre-tilt angle.
According to a first aspect of the present invention, there is provided a polyamic acid composition comprising a polyamic acid providing a polyimide resin giving a small residual voltage and a high voltage holding ratio, and a polyamic acid providing a polyimide resin giving a large pre-tilt angle, the ratio (weight ratio) of the former to the latter being 50/50 to 95/5.
According to a second aspect of the present invention, there is provided a polyamic acid composition comprising a polyamic acid A and a polyamic acid B, wherein the essential acid component of the polyamic acid A is an alicyclic tetracarboxylic dianhydride, the diamine component of the polyamic acid A is a diamine having at least one of the aromatic diamines represented by the following formula (1), the acid component of the polyamic acid B contains 50 mole % or more aromatic tetracarboxylic dianhydride, and the essential diamine component of the polyamic acid B is a diamine having a group enabling the pre-tilt angle of a liquid crystal to be increased on the side chain thereof, the ratio A/B of the polyamic acid A to the polyamic acid B being 50/50 to 95/5 (by weight). 
where, X is a divalent aliphatic group, each R is independently a hydrogen atom or CH3, and a and b are 1 or 2.
According to a third aspect of the present invention, there is provided a polyamic acid composition according to the second aspect, wherein the acid component of the polyamic acid A comprises an alicyclic tetracarboxylic dianhydride and an aromatic tetracarboxylic dianhydride.
According to a fourth aspect of the present invention, there is provided a polyamic acid composition according to the second aspect, wherein the diamine component of the polyamic acid B comprises at least one diamine having a group enabling the pre-tilt angle of a liquid crystal to be increased on the side chain thereof, and a diamine represented by the formula (1).
According to a fifth aspect of the present invention, there is provided a polyamic acid composition according to the second aspect, wherein the essential diamine component of the polyamic acid B comprises at least one of the diamines represented by the following formulas (2) and (3). 
where R is a hydrogen atom or an alkyl group having 1 to 12 carbon atoms, Y is a CH2 group, m is an integer from 0 to 2, A is a benzene ring or a cyclohexane ring, p is 0 or 1, z is an oxygen atom or a CH2 group, and n is 0 or 1. 
where, X1 is a CH2 group or an oxygen atom, each of R1 and R2 are individually a hydrogen atom, an alkyl group or a perfluoroalkyl group having 1 to 12 carbon atoms; at least one of R1 and R2 being an alkyl group or a perfluoroalkyl group having 3 or more carbon atoms; and n1 is 0 or 1.
According to a sixth aspect of the present invention, there is provided a polyamic acid composition according to the second aspect, wherein the diamine component of polyamic acid B comprises at least one of the diamines represented by formulas (2) and (3), and the diamine represented by formula (1).
According to a seventh aspect of the present invention, there is provided a polyamic acid composition according to the second aspect, wherein the acid component of the polyamic acid A comprises an alicyclic tetracarboxylic dianhydride and an aromatic tetracarboxylic dianhydride, and the diamine component of the polyamic acid B comprises at least one of the diamines represented by formulas (2) and (3), or a diamine comprising said diamine and the diamine represented by formula (1).
According to an eighth aspect of the present invention, there is provided a polyamic acid composition according to the second aspect, wherein the acid component of the polyamic acid A comprises an alicyclic tetracarboxylic dianhydride and an aromatic tetracarboxylic dianhydride, the ratio of the former to the latter being 90/10 to 10/90 (mole ratio), and the diamine component of the polyamic acid B comprises at least one of the diamines represented by formulas (2) and (3), or a diamine comprising said diamine and the diamine represented by formula (1).
According to a ninth aspect of the present invention, there is provided a polyamic acid composition according to the second aspect, wherein the main acid component of the tetracarboxylic dianhydride of the polyamic acid A is cyclobutane tetracarboxylic dianhydride, the acid component of the polyamic acid B contains 50 mole % or more pyromellitic dianhydride, and the diamine component of the polyamic acid B comprises at least one of the diamines represented by formulas (2) and (3), or a diamine comprising said diamine and the diamine represented by formula (1).
According to a tenth aspect of the present invention, there is provided a polyamic acid composition according to the second aspect, wherein the acid component of the tetracarboxylic dianhydride of the polyamic acid A comprises cyclobutane tetracarboxylic dianhydride and pyromellitic dianhydride, and the diamine component of the polyamic acid B comprises at least one of the diamines represented by formulas (2) and (3), or a diamine comprising said diamine and the diamine represented by formula (1).
According to an eleventh aspect of the present invention, there is provided a polyamic acid composition according to the second aspect, wherein the acid component of the tetracarboxylic dianhydride of the polyamic acid A comprises cyclobutane tetracarboxylic dianhydride and pyromellitic dianhydride, the acid component of polyamic acid B contains 50 mole % or more pyromellitic dianhydride, and the diamine component of the polyamic acid B comprises at least one of a diamine represented by formula (2) and a diamine represented by formula (3), or a diamine comprising said diamine and the diamine represented by formula (1).
According to a twelfth aspect of the present invention, there is provided a polyamic acid composition according to the second aspect, wherein the acid component of the polyamic acid A comprises pyromellitic dianhydride and cyclobutane tetracarboxylic dianhydride, the ratio of the former to the latter being 90/10 to 10/90 (mole ratio); and the acid component of the polyamic acid B containing 50 mole % or more of pyromellitic dianhydride, and the diamine component of the polyamic acid B comprises at least one of the diamine represented by formula (2) and a diamine represented by formula (3), or a diamine comprising said diamine and the diamine represented by formula (1).
According to a thirteenth aspect of the present invention, there is provided a polyamic acid composition according to the second aspect, wherein the acid component of the polyamic acid A comprises pyromellitic dianhydride and cyclobutane tetracarboxylic dianhydride, the ratio of the former to the latter being 90/10 to 10/90 (mole ratio), the diamine component of the polyamic acid A is a diamine represented by formula (1), where X is CH2 or CH2CH2, and each R is a hydrogen atom; the acid component of the polyamic acid B contains 50 mole % or more pyromellitic dianhydride, and the diamine component of polyamic acid B comprises at least one of a diamine represented by formula (2) and a diamine represented by formula (3), or a diamine comprising said diamine and the diamine represented by formula (1).
According to a fourteenth aspect of the present invention, there is provided an aligning film containing a polyimide resin obtained from a composition according to any one of the first through thirteenth aspects.
According to a fifteenth aspect of the present invention, there is provided a liquid crystal display element using an aligning film for a liquid crystal display element according to the fourteenth aspect.
According to a sixteenth aspect of the present invention, there is provided a TFT liquid crystal display element according to the fifteenth aspect using a liquid crystal composition mainly comprising the following compounds: 
where, R1 represents an alkyl group having 1 to 10 carbon atoms, in which optional, nonadjacent methylene groups may be substituted by xe2x80x94Oxe2x80x94 or xe2x80x94CHxe2x95x90CHxe2x80x94, and optional hydrogen atoms of the group may be substituted by fluorine atoms; X1 represents a fluorine atom, a chlorine atom, xe2x80x94OCF3, xe2x80x94OCF2H, xe2x80x94CF3, xe2x80x94CF2H, xe2x80x94CFH2, xe2x80x94OCF2CF2H, or xe2x80x94OCF2CFHCF3; each of L1 and L2 independently represents a hydrogen atom or a fluorine atom; each of Z1 and Z2 independently represents 1,2-ethylene, 1,4-butylene, xe2x80x94COOxe2x80x94, xe2x80x94CF2Oxe2x80x94, xe2x80x94OCF2xe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94, or a single bond; Ring B represents trans-1,4-cyclohexylene, 1,3-dioxane-2,5-diyl, or 1,4-phenylene in which hydrogen atoms may be substituted by fluorine atoms; and Ring C represents trans-1,4-cyclohexylene, or 1,4-phenylene in which hydrogen atoms may be substituted by fluorine atoms.
According to a seventeenth aspect of the present invention, there is provided a liquid crystal alignment material containing a polyamic acid composition according to the second aspect.